Dizziness and vertigo are common clinical problems rooted in abnormal perception of movement. Clinical testing of the vestibular system and understanding of vestibular physiology has historically focused on simple stimuli and reflex function. However, vestibular reflex function is poorly correlated with perceptual symptoms. In other sensory modalities perceptual testing is the first line of clinical investigation. For instane an evaluation of vision usually begins with having the patient report what they are able to see and evaluation of hearing begins with testing what the patient is able to hear. However, understanding of vestibular perception is not yet at a state where perceptual testing is clinically meaningful. Most of the previous work on vestibular perception has focused on measurement of perceptual thresholds - essentially determining if someone can sense the direction of an isolated movement in a dark room. This situation does not address the more clinically relevant situations in which each movement occurs in the context of ongoing motion which include both translation and rotation components as well as other sensory stimuli such as motor and proprioception. The current proposal aims to advance understanding of human motion perception into a clinically and physiologically relevant arena by examining this perception in an appropriate context. The project will also test psychometric methods to determine which is most appropriate for measurement of vestibular biases. Some combinations of head rotation and translation are more benign and likely to occur during ambulation while others may be indicative of a perturbation that may lead to a fall. The project will look how proceeding movements influence motion perception such that a movement that is similar to a previous one is less likely to be perceived. It is common for people to make large voluntary movements without becoming disoriented, such as looking to the side to talk to a friend while walking. This may be easy for us because we perceive our motion in a body-centered reference frame rather than head-centered one, even though the vestibular organs are in the head. How this sensory transformation occurs will be investigated for heading estimation. Visual stimuli also provide a powerful cue to self-motion perception, especially when the visible motion involves a large part of the visual field and persists for several seconds. This perception is often referred to as vection and may also be related to motion sensitivity in some patient populations. A method of quantifying vection by comparing it with actual motion will be developed. This will be useful for determining if vection and self-motion are perceived equivalently as well as proving an objective method of assessing visual motion sensitivity.